1. Field of the Invention
The present invention relates to a control apparatus and method for an internal combustion engine capable of switching a combustion mode between a spark ignition combustion mode and a compression ignition combustion mode.
2. Description of the Related Art
Hitherto, as a combustion method capable of simultaneously attaining improvements of fuel efficiency and an exhaust gas performance in an internal combustion engine (hereinafter referred to as engine), there is known a combustion method (hereinafter referred to as HCCI combustion mode) for igniting a uniform mixture by itself by compressing the mixture with a piston.
As compared with a combustion method (hereinafter referred to as SI combustion mode) involving flame propagation combustion for igniting a mixture with spark ignition of a spark plug, the HCCI combustion mode can improve thermal efficiency with a high compression ratio, can reduce a cooling loss with rapid combustion, and can also reduce a pumping loss, thereby reducing a fuel consumption amount, and can restrain nitrogen oxides (hereinafter referred to as NOx) from being generated with low temperature combustion of a uniform mixture high in the air fuel ratio or the exhaust gas recirculation (EGR) ratio.
In the HCCI combustion mode, the HCCI combustion is realized by keeping a part of a burnt gas generated by the previous combustion cycle remaining (hereinafter referred to as internal EGR), thereby increasing a mixture temperature. However, when the engine is in an operation area on a relatively low load and low revolution side, an increase in temperature of the mixture with the internal EGR is insufficient, and thus the HCCI combustion cannot be realized. Therefore, and the combustion mode needs to be switched to realize the SI combustion in this operation area.
As a technology for switching the combustion mode, Japanese Patent No. 4438792 discloses a technology for switching the combustion mode through mixed combustion of increasing the internal EGR, and increasing the pressure and temperature of a remaining mixture by a combustion pressure of flame propagation combustion by spark ignition, resulting in compression ignition.
However, in the combustion mode switching method disclosed in Japanese Patent No. 4438792, inactive gasses included in the mixture also increase as a result of the increase in the internal EGR, and hence the flame propagation combustion by the spark ignition hardly occurs, resulting in a combustion fluctuation and a misfire. Therefore, when the internal EGR more than a predetermined amount is carried out, stable mixed combustion cannot be realized due to an increase in the combustion fluctuation and the misfire, resulting in such a problem that the combustion mode cannot be switched.
Moreover, as the internal EGR increases, a combustion period extends, and hence the ignition timing needs to be advanced for optimal thermal efficiency. In this case, as the ignition timing is separated from the compression top dead center (hereinafter referred to as compression TDC) more, the pressure and temperature of the mixture at the ignition timing decrease. Further, the combustion temperature decreases as the internal EGR increases, and hence when the ignition timing is advanced as described above, the remaining mixture does not reach a pressure and a temperature for the compression ignition in the flame propagation combustion by the spark ignition, resulting in such a problem that the combustion mode cannot be switched.